C. John LoNorthrop Grumman Aerospace Systems, Redondo Beach, California

John R. Brophy, M. Andy Etters, and Rajeshuni RameshamJet Propulsion Laboratory, Pasadena, California

William R. Jones, Jr.Sest Inc., Middleburg Heights, Ohio

Mark J. JansenUniversity of Toledo, Toledo, Ohio

Use of Cumulative Degradation Factor Prediction and Life Test Result of the Thruster Gimbal Assembly Actuator for the Dawn Flight Project

NASA/CR—2009-215681

October 2009

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C. John LoNorthrop Grumman Aerospace Systems, Redondo Beach, California

John R. Brophy, M. Andy Etters, and Rajeshuni RameshamJet Propulsion Laboratory, Pasadena, California

William R. Jones, Jr.Sest Inc., Middleburg Heights, Ohio

Mark J. JansenUniversity of Toledo, Toledo, Ohio

Use of Cumulative Degradation Factor Prediction and Life Test Result of the Thruster Gimbal Assembly Actuator for the Dawn Flight Project

NASA/CR—2009-215681

October 2009

National Aeronautics andSpace Administration

Glenn Research CenterCleveland, Ohio 44135

Prepared under Contract NNC07JF14T

Acknowledgments

This research was carried out in part, at the Jet Propulsion Laboratory, California Institute of Technology in Pasadena, California, under Contract Task Plan Number 40–12079 with the National Aeronautics and Space Administration.

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USE OF CUMULATIVE DEGRADATION FACTOR PREDICTION AND LIFE TEST RESULT OFTHE THRUSTER GIMBAL ASSEMBLY ACTUATOR FOR THE DAWN FLIGHT PROJECT

ABSTRACT

Dawn [1]-[3] is the ninth project in NASA’s DiscoveryProgram. The Dawn spacecraft is being developed toenable the scientific investigation of the two heaviestmain-belt asteroids, Vesta and Ceres. Dawn is the firstmission to orbit two extraterrestrial bodies, and thefirst to orbit a main-belt asteroid. The mission isenabled by the onboard Ion Propulsion System (IPS) toprovide the post-launch delta-V [1]. The three IonEngines of the IPS are mounted on Thruster GimbalAssembly (TGA) [2], with only one engine operatingat a time for this 10-year mission. The three TGAsweigh 14.6 kg [1].

Figure 1 – Dawn Flight System Configuration

Figure 2 – IPS Thrusters Configuration

Each Ion Engine/TGA is single string with noredundancy. System redundancy is provided byrequiring two out of the three Ion Engine/TGA tooperate for the full mission duration. The TGA is ahexapod design with two articulating crank-armactuators, changing the effective length of the legs andproviding pointing control for the Ion Engine. Adrawing and photo of the TGA hexapod gimbal [2] isshown in Figures 3 and 4.

Figure 3 – Dawn Hexapod Gimbal

Figure 4 – TGA during assembly (May 2006)

The input to the crank arm actuator is a vendorprovided (Starsys Inc., Louisville, CO, USA)gearmotor, with a new two phase 45° stepper motordesign driving a two stage (4.333 x 4.333) planetarygearbox. The gearbox has Mars Exploration Rover(MER) heritage and was used in the wheel steeringactuators. The gearmotor output then drives a standard

C. John Lo Northrop Grumman Aerospace Systems

Redondo Beach, California 90278

John R. Brophy, M. Andy Etters, and Rajeshuni Ramesham Jet Propulsion Laboratory

Pasadena, California 91109

William R. Jones, Jr. Sest, Inc.

Middleburg Heights, Ohio 44130

Mark J. Jansen University of Toledo Toledo, Ohio 43606

NASA/CR—2009-215681 1

Harmonic Drive System (HDS) (Type 17 harmonicdrive with a 100:1 gear reduction) using 52100 bearingmaterials in the wave generator bearing. The outputangle is 0.024 degree per step. The output stage isdesigned and built by JPL. The harmonic drive outputis then connected to the crank-arm by a duplex 440CStainless Steel (SS) pair bearing (Figure 5).

Figure 5 – Dawn TGA Actuator

The TGA crank arm actuator is a new design. Thelubricant life prediction as presented at the TGACritical Design Review (CDR) in May 2004 did notprovide sufficient rationale as to why the lubricationwill survive the predicted mission life time. Once thisdeficiency was identified, the Cumulative DegradationFactor [4] (CDF) technique was implemented todetermine whether the design could meet the missionlifetime requirement before completing thegearmotor/actuator design and assembly. As a resultof this study, numerous changes, including missionduty cycle definition, motor rotor bearing preloadvalue and additional amount of lubricant applied to therotor bearings were implemented to assure missionsuccess, plus a successful life test. In addition, arevised stepper motor design, using external resistorsto stabilize the system resistance, was incorporated inthe same time period to work with the existing motorcontrol logic.

1. EXPERIMENTAL

A Spiral Orbit Tribometer (SOT) (Figure 6) was usedto study the lifetime of the maraging alloy steel,Vascomax® C-300 gear materials used in the two-stageplanetary gearbox. No data existed prior to this studyon the new Vascomax C-300 material when used in amechanical moving assembly (MMA) for space flight.The SOT [5] measures lubricant consumption in theboundary lubrication regime to provide a comparativetribological consumption rate quickly with differentmaterial combinations, contact stresses andtemperatures.

Figure 6 – The Spiral Orbit Tribometer

Custom Vascomax C-300 test and guide plates werefabricated for the SOT, and a 440C SS 1/2 inch (12.7mm) diameter ball was used as Vascomax C-300 ballswere not available. The test plates were run withBraycote® 601EF and 602EF greases, Brayco 815Zand Pennzane® oils with the 440C SS test balls. TheseSOT tests are then compared with 440C SS testplate/ball used as reference. The purpose of this test isto investigate whether the Vascomax C-300 materialhas any accelerating effect on the lubricantconsumption rate as seen by the 17-4 PH SS withPennzane in the Microwave Limb Sounder AntennaActuator Assembly (AAA) [4]. The SOT tests wererun in ultrahigh vacuum (< 10-6 Pa). A normalizedlubricant lifetime was determined by dividing thenumber of ball orbits at failure (friction coefficient of0.28) by the amount of lubricant deposited on the ball.This yields a lifetime in orbits/µg of lubricant and isused to compare different combinations and twotemperatures.

A dedicated crank arm actuator Life Test Unit (LTU)was initially used in one of the two arms in theQualification TGA. The LTU was subjected to 6-1/2thermal cycles between +120°C to -70°C in GN2 a tqualification temperature during actuator level testing,then installed in the TGA for the qualificationvibration test to qualification level and duration (2minutes/axis). Due to a problem with the specialwelded Resistoflex fitting adaptors during vibrationtesting, and subsequent retest, the TGA was subjectedto a total of 12 minutes of vibration testing, twice thenormal duration. After vibration, the LTU wasremoved from the qualification TGA and replaced withan EM actuator for further TGA comprehensiveperformance testing (CPT). A post-vibration 1-1/2thermal cycle test was performed to verify the

NASA/CR—2009-215681 2

performance of the LTU. This testing also consisted ofMotor Threshold Voltage (MTV) and running torquetesting at temperature and ambient. After successfullycompleting these tests, the LTU was setup in a bell jarvacuum chamber and programmed for a long durationlife test, using precise power control to bring thethermally isolated LTU actuator to between +55°C to+89.5°C running temperature in a set profile byheating the winding and the two external resistors.

Periodic measurements using Lab View® softwaremeasures the MTV automatically to monitor changesto the internal running friction, giving an indication asto the state of the lubricant inside the LTU. The LTUlife test was completed in May 2006, and the LTUdisassembled at JPL. The gearmotor was returned tothe vendor for further disassembly.

2. RESULTS

2.1 Cumulative Degradation Factor Prediction

The Cumulative Degradation Factor (CDF) analysisinitially indicated “Very Low Probability of Success”as designed at the CDR. The proposed lubricants forthe DAWN TGA are all based on Brayco 815Z, linearperfluoropolyether oil. This oil and its greasevariations have been the lubricants of choice for spacemechanisms for many years. The gears and gearboxwas to be lubricated with Braycote 602EF grease.Braycote 601EF grease and Brayco 815Z oil were tobe used in the motor, motor bearings, and output shaftbearings. Both greases contain a PTFE thickener andBraycote 601EF also contains a bentonite claycorrosion inhibitor and sodium nitrite rust inhibitor.The Braycote 602EF only contains MoS2, a solidlubricant. . The TGA ball bearings are made of 440Cstainless steel and the gears are made of Vascomax C-300, maraging steel containing Ni, Co, and Mo.

After examining the relative number of stress cycles ofall TGA bearings, it was determined the motorbearings were at greatest risk because they undergo themost cycles. Planet bearings see reduced cyclesbecause of the various gear ratios in the TGA.Therefore, calculations are based only on the motorbearings.

When the 10% mission duty cycle, proposed by JPL,was used in the calculation of the CDF, it clearlyindicated that the lubricant in the motor bearing wouldnot survive. With full cooperation from the gearmotorvendor, a different design was implemented to lowerthe motor rotor bearing preload, to 25% of the initialvalue at 1.0 ± 0.25 lbf. However, this redesign CDFcalculation still yielded the same failure prediction.

For a five-year lifetime, at a 10% duty cycle, the motorbearings will experience approximately 100 millionrevolutions. This is based on 4,380 hours of operationat 6.25 revolutions/sec. The bearings are small(0.375” OD), 440C SS, Conrad deep groove bearingswith eight 1/16” diameter balls. The ball-raceconformity is 56% and the free contact angle is 15.6degrees. The final selected preload was 1.00 ± 0.25 lbf.Assuming a maximum preload of 1.25 lbf, the meanHertzian stress is 113 ksi. For the eight ballcomplement, one shaft revolution will yield four ballpasses. Therefore, 100 million revolutions yields 400million ball passes. Multiplying this value with themean Hertzian stress yields a CDF of approximately4.5 x 1013 ball passes-psi. For comparison, measuredCDF values for several bearing life tests appear inTable 1.

Table 1, Measured Cumulative Degradation Factor

Only measured CDF by testing in vacuum were usedfor comparison as even a small amount of moisturefrom GN2 or LN2 can greatly reduce the lubricantconsumption rate [7].

This prediction was then used as the tool to convincethe Dawn project to accept the spacecraft vendor’s(Orbital Science Corp., Dulles, VA) mission duty cycleestimate of 1%. With this realistic value of 1%mission duty cycle, and in combination with the lowestpractical rotor bearing preload, the prediction using theCDF technique changes to “High Probability ofSuccess” for meeting the updated mission requirementwith a revised CDF of 4.5 x 1012 bp-psi. This revisedCDF is in the midrange of Lockheed Martin’s failurerange of 2.2 to 8.7 x 1012 bp-psi based on bearing lifetests [6].

Although the CDF for bearing tests with Pennzane oilformulations is an order of magnitude greater than forBrayco 815Z, Pennzane was not chosen for the MMAsince the unit would be operating at elevatedtemperature. The vapor pressures of Pennzaneformulations are about two orders of magnitude greaterthan Brayco 815Z at temperatures approaching 90°C.The actual consumption and evaporation rates shouldbe measured with the SOT and in actual bearing lifetests at these higher temperatures. The operating

NASA/CR—2009-215681 3

temperature for the Dawn TGA, while the Ion Engineis running, is predicted to be between 55 to 89.5°C,and is beyond life test and on-orbit experience forPennzane formulations. It was felt that the risk ofusing Pennzane at this temperature is high without athorough investigation and test program.

2.2 Vascomax C-300 Consumption Rate

The SOT results (Figure 7) indicated Vascomax C-300did not have an accelerated lubricant consumption ratewith Braycote 601EF grease. However, the data didindicate a reduced lifetime on both Vascomax and440C SS with 602EF when compared to 601EF. Thegearmotor vendor was quickly directed to change fromthe 602EF lubricant (used on MER) to 601EF tomaximize life of the gears used on Dawn. Also, allMMA processes for the Dawn TGA at JPL use the601EF grease with Braycote 600 used for greaseplating.

Figure 7 – Vascomax C-300 SOT Test(< 10 -68 Pa, 40 RPM, 1.5 GPa mean Hertzian Stress)

2.3 Modifications to the Stepper Motor

The initial gearmotor specification omits the allowablemaximum torque and the first generation stepper motorwas designed to the maximum capability of the two-stage planetary gearbox. This excessive torque willdamage the harmonic drive, producing a calculatedtorque of more than 1,500 in-lbs. The gearmotorspecification was then clarified with clear definition ofthe winding configuration (shorting of un-poweredwinding), two missions specified fixed pulse width at15 and 20 ms, and allowing the addition of externalresistors to stabilize motor current (constant voltagedriver) over the wide temperature range. The revisedmotor also incorporated the reduced preload afterdelivery of the first Engineering Model (EM).

2.4 EM 1 Configuration

The first EM had the original 4 lbf preload and thestandard (larger -1) gear with the revised steppermotor. The rationale is that the first path finding EMshould be built as quickly as possible with existinghardware. This EM was used in the thermal dynochamber setup, resulting in the discovery of gearbinding at temperatures below –55°C.

2.5 EM2 Configuration

The second EM, EM2, was initially intended to be theLTU. The rotor bearing preload was lowered to 2 lbfeasily by using different thickness shim washers. EM2was assembled with the smaller -2 gears with lighthand lapping. The CDF with the 2 lbf was in anacceptable range, but on the high side of the failurerange. EM2, with 2.0 lbf and -2 gears, was delivered toJPL and went through the full qualification thermaldyno test profile for 6-1/2 thermal cycles whilemodifications to the rotor bearing preload was beingengineered. This EM2 produced acceptableperformance, but with more reduction in output torqueat temperatures below –55°C, indicating high internalfriction.

2.6 LTU Configuration

Subsequently, the target rotor bearing preload waslowered even further, to 1.0 ± 0.25 lbf for the third unitbeing built. This unit, the LTU, used a differentpreload bearing cup design, with each wave springmeasured and the cup match machine to bring thepreload to the design range. The rotor bearings aregrease plated and four additional drops of Brayco 815Zoil added to every other ball for each rotor bearing.The gearbox used the smaller -2 gears with very lightlapping to a newly defined “end play” target range forthe gearmotor output. After grease plating the gearsand the bearings in the planetary gearbox, additionalgrease strips with Braycote 601EF were applied on theinside of the gearbox housing. After bake out, theLTU was brought down to –108°C cold survivaltemperature, then 6-1/2 thermal cycles between –70°Cto 120°C, with dyno runs at the first and last thermalcycle. Below (Figure 8) is the thermal dyno test resultfor the LTU in April 2005:

NASA/CR—2009-215681 4

Figure 8 – LTU Pre-Vibe Thermal Dyno Test, 50 pps

The pulse width is commandable from the ground tofull pulse width (20 ms) if needed, with an increase ofapproximately 30 to 40 in-lb of output torque from the15 ms value.

Post-Vibe thermal dyno test of the LTU in July 2005produced similar torque output, MTV and nomovement due to vibration by measuring the numberof steps to actuate the microswitches at the hardstopposition.

2.7 HD Wave Generator Bearing SeparatorOrientation

An EM TGA was built in the summer of 2004, withflight configuration titanium struts, actuator bracket,and all flight like hardware as the vibration test fixtureto qualify the three flight ion engines. Instead of agearmotor, a simple mass model took its place, withmanual crank to drive the input to the wave generatorof the harmonic drive. The separator was installed onthe “closed” side of the flex spline of the harmonicdrive. In order to keep the separator from popping offwhen load was applied, a set of custom shims and alarge 400 series SS washer was used to keep theretainer in place. However, during assembly of theactuators with the running gearmotors from the firstEM on, the retainer was flipped and installed on the“opened” side of the flex spline on the wave generatorbearing. This simple and seemingly innocent changeresulted in a situation that, when the wave generatorbearing is loaded, the retainer is pushed outward

against a titanium gearmotor output adaptor hub.Titanium could, and in this case did, acceleratedegradation of the Braycote lubricant. Futuregearmotors should fabricate this output adaptor hubwith 440C SS materials to prevent this defect so thatthe retainer, with the Braycote lubricant, will slide on aknown material that will not accelerate break down ofthe lubricant.

2.8 HD Wave Generator Bearing Materials

The off-the-shelf HDS harmonic drive uses 52100bearings. Much effort was spent on run in anddifferent techniques to prevent the bearing fromrusting after precision cleaning. Future gearmotordesigns should specify 440C SS bearings to simplyprocessing. The additional lead time using the 440CSS bearing is well worth the wait.

2.9 External Resistive Load

At TGA CDR, the resistive load was estimated at morethan 50 in-lb. With a revised crimping procedure forthe spherical ball bearings at the end of the struts,redesigned routing for the three 1/8” SS Ion Enginefeed lines into a “cloth spring” configuration, andrepositioning of the high voltage lines, the measuredresistive load at room temperature has been reduced to6 in-lb maximum. The actuator consistently deliveredmore than 220 in-lb of output torque, providing morethan adequate torque margin with a fixed 15 ms pulsewidth from 0 to 50 pulses per second (pps).

NASA/CR—2009-215681 5

2.10 Detailed Lubrication Scheme for the Actuator

The two Starsys motor bearings are initially greaseplated, then an additional 4 drops of Brayco 815Z oil(15 ± 3 mg total for each bearing) is applied to eachbearing, the planetary gears and bearings are grease-plated with Braycote 600 grease, and four Braycote601EF grease strips applied on the inside of thegearbox housing. On the JPL side, the harmonic drivewave generator bearing, flex spline, circular spline andthe duplex bearing pairs are grease plated withBraycote 600, then all lubricated by a 50/50 mix ofBrayco 815Z oil and Braycote 601 grease. Allbearings are packed at 60 to 75% of free volume withthis 50/50 mix. The inside of the flex spline hasapproximately 400 to 600 mg of this mix to provide areservoir around the sliding surface for the wavegenerator bearing outer race. The circular spline andflex spline gears are also filled with this 50/50 mix.

2.11 Test as You Fly, Fly as You Test

The LTU is exactly the same as all flight units. It is“Baked and Shaked” before life test. Also the LTU istested in vacuum instead of GN2, at the same step rateplanned for flight. The temperature profile is based onprediction by integrated thermal analysis with thespacecraft contractor.

2.12 Life Test Setup

The main objective of this test was to temperaturecycle a DAWN TGA actuator in vacuum (with targetrange between 2 to 7 x 10-5 Torr pressure during lifetesting, and vacuum level at least to 10 x 10-5 Torr isacceptable during the beginning of the bake-out/pumpdown stage in order to expedite life test), automaticdirection switching at end of travel, consistent with theset temperatures such as three ranges, automated datahandling to monitor temperature, weekly inspection ofdata. Duration is approx. 3 months for a 3X life testrun (demonstrate 1% mission duty cycle, 5 yearsmission life per TGA, 2 TGA functional (out of 3TGA) for the 10 year DAWN mission life time).Successful demonstration of the 3X life test run ormore will meet the DAWN project requirement for lifetesting of the DAWN actuator.

Figure 9 - Dawn LTU prior to Life Test

Figure 10 - Dawn LTU in Test Lab

By monitoring the power to run the motor, an increasein power could give a clear indication of the health ofthe lubricant inside the motor. The point of failure wasdetermined by measuring the Motor Threshold Voltage(MTV) which is the lowest voltage at which the motorwill start turning, as evidenced by a drop in current.An initial measurement of the MTV was made at thestart of the test of the LTU, and End of Life (EOL) forthe LTU (as indicated by an increase of power requiredto start the motor due to lubricant failure) is definedarbitrarily as 150% of the initial MTV value. Even atthe EOL MTV, the value is estimated to be still belowthe minimum voltage of 22.0 Vdc. The actuator willcontinue to function at this arbitrary EOL, and wellbeyond until the increase in friction of the bearings andgears exceeds the power generated by the gearmotor.

2.13 Temperature Profile

The life test was carried out using the temperatureprofile shown in the figure 11. The profile shown inthe figure corresponds to one life cycle. The actuatorwas first rotated in the clockwise (CW) direction (halfcycle) for 245 hours (=168 hrs at 58°C to 72°C + 44hrs at 78°C to 82°C + 33 hrs at 85.5°C to 89.5°C), thenthe actuator was rotated for another 245 hours in thecounter clockwise (CCW) direction following the same

NASA/CR—2009-215681 6

profile to complete one life cycle. This process hasbeen repeated seven times (7X) to meet the re-definedproject requirement.

Figure 11– Temp. Profile Based on Mission Prediction

2.14 Life Test Result

As a result of an action assigned to the Dawn Projectafter the stand-down in March 2006, the NASAindependent review team directed that the LTU shouldaim for a 7X life test, instead of 3X, to proverobustness of this new system. At the elevatedtemperature, which seems to provide more life (whichis opposite from the SOT test data), the first serioussign of lubricant degradation appears at 4.6X life, withthe gearmotor overpowering this increased internalresistance, completing the 7X life with abundanttorque margin (with MTV at 12 Vdc and a minimumsupply voltage of 22 Vdc). Subsequent disassembly ofthe LTU and the gearmotor indicates the Braycotelubricant initially breaks down at the harmonic drive(HD) wave-generator separator/titanium hub interface,with the lubricant in the rotor bearing in slightlydegraded but usable condition.

Figure 12 - MTV During Life TestThe three spikes prior to 4.6X life were attributed totest equipment errors. After reset, the MTV returnedto the nominal range. At 4.6X life, there is clearindication that the lubricant has broken down, resultingin erratic and a gentle upward trend, accumulating to5.5X life. The LTU seems to recover but started theupward trend again. Post life test lubricant analysis isshown in Table 2.

Table 2, HD and Output Bearing Post-Life Analysis

The HD WG bearing separator had rubbed on thetitanium adaptor hub, resulting in accelerateddegradation of the Brayco lubricant, with end oflubricant life reached before the motor rotor bearing.The HD gears are in excellent condition, with themachine marks on the gears still visible.

NASA/CR—2009-215681 7

Figure 13 -LTU HD WG Bearing after Life Test

The gearmotor output bearing, in close proximity withthe HD, was contaminated by the degraded HDlubricant. The stepper motor rotor bearing, separatedby the gearbox, is actually in excellent condition, witha clear film of lubricant around the balls.

Figure 14-LTU Gearmotor Components after Life Test

The chemical analysis report by JPL’s AnalyticalLaboratory appears in Table 3.

Table 3, Gearmotor Output and Rotor Bearing Post-Life Analysis

2.15 Post Life Test Analysis

Post-life test gearmotor performance indicates thegearmotor is actually fully “run-in”, with gear

efficiency approaching 98%. Also, the end play at thegearmotor has increased by 0.16 deg, from an initial0.7 deg to 0.86 deg.

Table 4, Gearmotor Pre and Post Life Test Data

3. TEST SUMMARY

At 4.6 times life, the CDF for the LTU is 19.3 x 1012

bp-psi. At the end of life test, with MTV at only 12Vdc, the CDF is 29.4 x 1012 bp-psi. This LTU hasexceeded the comparable Lockheed Martin life testdata. The use of additional oil may have had asignificant effect on lifetime. Although lubricantconsumption increases with increasing temperature,higher temperatures can improve lubricant supply dueto higher resupply rates. This phenomenon has beenobserved during life tests with Braycote 601EF greasefor the Space Shuttle Body Flap Actuator bearings [8].In these tests, bearing lifetimes at 60°C were greaterthan at 23°C.

4. CONCLUSIONS

New systems should always be analyzed with the CDFtechnique to predict lifetime. After that, design andmission planning should be performed within thelifetime predicted. The LTU running at highertemperature actually produced longer life thanpredicted by life test data at room temperature.Lubricant failure occurred at a CDF of 19.3 x 1012 bp-psi. Vascomax C-300 did not accelerate lubricantbreakdown, as evidenced by SOT and LTU post-lifeanalysis. The LTU was life tested in high vacuum,providing validity to the test data.

5. LESSONS LEARNED

Implementation of the above techniques increased taskcost since these were not in the original plan.However, timely changes to hardware addedsubstantial value to the project with a robust system.Also, a clear understanding of the tribological effects

NASA/CR—2009-215681 8

with un-tested materials minimizes schedule risk incase of a life test issue. This allowed completion of asuccessful life test program of “new” hardware,without jeopardizing the Dawn project for launch.

Space mechanism designers utilizing lubricatedmechanical moving assemblies (MMAs) made ofalloys other than conventional bearing steels, such as440C SS or 52100, need to verify that missionlifetimes will not be compromised by adverselubricant/alloy interactions.

. REFERENCES

[1] Brophy, John R., E.P., “Development and Testingof the Dawn Ion Propulsion System”, AIAA 2006-4319, 42nd AIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibit, 9-12 July, 2006.

[2] Brophy, John R., et al., “Status of the Dawn IonPropulsion System”, AIAA 2004-3433, 40th

AIAA/ASME/SAE/ASEE Joint Propulsion Conferenceand Exhibit, 11-14 July, 2004.

[3] Brophy, John R., et al., “Implementation of theDawn Ion Propulsion System”, AIAA 2005-4071, 41st

AIAA/ASME/SAE/ASEE Joint Propulsion Conferenceand Exhibit, 10-30 July, 2005.

[4] Jones, William R., et al., “The Effect of 17-4 PHStainless Steel on the Lifetime of a PennzaneLubricated Microwave Limb Sounder AntennaActuator Assembly Ball Screw for the AURASpacecraft, Proc. 11th ESMATS Symp., SP-591,Lucerne, Switzerland (2005)

[5] Pepper, S.V. and Kingsbury, E.P., “Spiral OrbitTribometry-Part 1: Description of the Tribometer”,Trib. Trans., 46, 1, pp 57-64 (2003).

[6] Bazinet, D.G. et al., “Life of Scanner Bearingswith Four Space Liquid Lubricants”, Proc. 37th

Aerospace Mech. Symp., NASA CP-2004-212073,Galveston, TX (2004).

[7] Pepper, S.V., “Effect of Test Environment onLifetime of Two Vacuum Lubricants Determined bySpiral Orbit Tribometry”, Proc. 38th Aerospace Mech.Symp., NASA CP-2006-214290, Williamsburg, VA(2006).

[8] Jett, T.R., et al., “Space Shuttle Body Flap ActuatorBearing Testing for NASA Return to Flight”, , Proc.38th Aerospace Mech. Symp., NASA CP-2006-214290,Williamsburg, VA (2006).

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6. AUTHOR(S) Lo, C., John; Brophy, John, R.; Etters, M., Andy; Ramesham, Rajeshuni; Jones, William, R., Jr.; Jansen, Mark, J.

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5f. WORK UNIT NUMBER WBS 431731.04.01.03

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Northrop Grumman Aerospace Systems 1 Space Park Redondo Beach, California 90278

8. PERFORMING ORGANIZATION REPORT NUMBER E-17052

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) National Aeronautics and Space Administration Washington, DC 20546-0001

10. SPONSORING/MONITOR'S ACRONYM(S) NASA

11. SPONSORING/MONITORING REPORT NUMBER NASA/CR-2009-215681

12. DISTRIBUTION/AVAILABILITY STATEMENT Unclassified-Unlimited Subject Category: 18 Available electronically at http://gltrs.grc.nasa.gov This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT The Dawn Ion Propulsion System is the ninth project in NASA’s Discovery Program. The Dawn spacecraft is being developed to enable the scientific investigation of the two heaviest main-belt asteroids, Vesta and Ceres. Dawn is the first mission to orbit two extraterrestrial bodies, and the first to orbit a main-belt asteroid. The mission is enabled by the onboard Ion Propulsion System (IPS) to provide the post-launch delta-V. The three Ion Engines of the IPS are mounted on Thruster Gimbal Assembly (TGA), with only one engine operating at a time for this 10-year mission. The three TGAs weigh 14.6 kg. 15. SUBJECT TERMS Space tribology

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT UU

18. NUMBER OF PAGES

15

19a. NAME OF RESPONSIBLE PERSON STI Help Desk (email:help@sti.nasa.gov)

a. REPORT U

b. ABSTRACT U

c. THIS PAGE U

19b. TELEPHONE NUMBER (include area code) 443-757-5802

Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39-18